(1) Field of the Invention
The present invention relates to a method, a system and a local network switching unit for providing access for a mobile communication terminal and the moved mobile communication terminal to an Internet network via wireless local networks. The invention relates in particular to a method and system for the efficient handover of the access to an Internet network to a mobile communication terminal which is moved from a first wireless local network to a further wireless local network.
(2) Description of the Related Art
Wireless local networks, so-called Wireless Local Area Networks (WLAN), serve as communication systems for communicating with other communication units for users with mobile communication terminals. Wireless local networks are used primarily as access networks for access to a fixed network, and in particular for access to the Internet. In a WLAN, data are transmitted in each case via an air interface. Data are thereby transmitted from one point to another by means of electromagnetic waves, in particular radio or infrared waves. A typical WLAN comprises at least one point of access, or so-called Access Point (AP), and at least one wireless communication terminal, for example a laptop or palmtop computer, for instance a so-called Personal Data Assistant (PDA), or a mobile telephone. An access point comprises at least one transmitter/receiver and an antenna for data exchange with the wireless communication terminals via an air interface. A device configuration embodying such an access point will be designated in this text as an access point unit. An access point unit is normally assigned a local network switching unit, which local network switching unit is typically connected to the access point unit via a physical network. A local network switching unit has the task, among others, of enabling network connections between the communication terminals existing in a wireless local network and further communication terminals connected to the local network switching unit. However, a local network switching unit can also enable e.g. network connections between the communication terminals existing in a wireless local network. Local network switching unit and access point unit can be combined into a single device. Such a device will be designated in this text as a switching device. An access point unit is normally set up at a fixed location, data being able to be exchanged only with communication terminals located within the range of this access point unit. The range of an access point unit depends upon various factors such as the access point unit itself, the environment and the mobile communication terminal. To enable access via wireless local networks in a larger area, a corresponding number of access point units are distributed over this larger area in such a way that a mobile communication terminal has access to at least one of the access point units from as many points as possible of this larger area. For a mobile communication terminal moving out of the range of a first access point unit into the range of a second access point unit, the access of the mobile communication terminal must be switched over from the first access point unit to the second access point unit. This switching step is also designated as a “WLAN handover,” whereby, in other words, the changeover takes place from the first wireless local network, assigned to the first access point unit, to the second wireless local network, assigned to the second access point unit. A WLAN handover must be carried out as efficiently as possible, it being meant by efficient that the access of the mobile communication terminal may not be interrupted, or only as briefly as possible. An efficient WLAN handover is of especially great importance for applications such as IP (Internet Protocol) telephony. For efficient WLAN handovers there exist at the present time only solutions for wireless local networks within locally limited zones, in particular for wireless local networks of a locally limited zone of the same network provider, especially for wireless local networks inside the same building or building complex. An efficient WLAN handover between, for example, wireless local networks spread out over larger geographic regions or between different network providers, is, on the other hand, an unsolved problem. Apart efficiency, a WLAN handover should take place in an as robust manner as possible. Robustness of a WLAN handover is partly connected to efficiency of a WLAN handover. When a mobile terminal is just on the border of the ranges of two different access points, it may become difficult to decide which of these access points shall be used. This may lead to a situation where the mobile terminal connects to the first access point, then detects a better signal quality of the second access point and therefore starts to register to the second access point. While establishing a connection to the second access point, the signal quality for the first access point may have increased again and the mobile terminal may therefore start again to register with the first access point. If efficiency of a WLAN handover is low, also robustness for connecting to a WLAN in such a situation as is low. Robustness may be even more degraded due to the fact that some central server behind the access points has to manage connections from the mobile terminal to the Internet. For managing such connections, rerouting of data streams may be necessary. However, when a mobile terminal switches its registration between different access points back and forth, the design of a robust central server for managing connections from the mobile terminal to the Internet becomes a difficult task. A further disadvantage in the state of the art is handling of a WLAN handover between different network providers. In the state of the art, when a mobile terminal moves from an access point of a first network provider to the access point of a second network provider, the mobile terminal has to be fully registered to the network provider again. Even if different network providers would like to share access points in between each other, such that customers of a first network provider may benefit from network resources of a second network provider, such a sharing of access points is not possible in the state of the art.
The document RFC 2002, October 1996, (the document is also known as “Mobile IP”) of the Internet Engineering Task Force (IETF) describes a protocol for access to the Internet for mobile nodes. A mobile node thereby has two IP addresses. The first IP address is the so called home address, which is assigned to the mobile node in a fixed way. The second IP address is the so-called care-of address, which is newly assigned again in each case during a change from a first physical network to a further physical network. The home address is made known to the Internet by a so-called home agent. Data which are intended for the mobile node are first sent to the home agent, who forwards these data to the current care-of address of the mobile node. Mobile IP makes it possible for a mobile node to be able to switch between physical networks of the same kind (such as e.g. from a first ether network to a further ether network) as well as to be able to switch between physical networks of a different kind (such as e.g. from an ether network to a WLAN). With mobile IP a mobile node can also switch from a first wireless local network to a further wireless local network, a change between different network providers also being possible. With mobile IP it is not possible, however, to achieve a sufficiently efficient WLAN handover. With mobile IP it is also not possible to provide a robust WLAN handover. And finally, with mobile IP it is also not possible that network providers may share network resources in between each other.
In “Application-Layer Mobility Using SIP” (ACM SIGMOBILE Mobile Computing and Communications Review, Volume 4, Number 3, 2000, 47-57), Schulzrinne et al. have proposed a SIP (Session Initiation Protocol) based handover. A mobile node receives a beacon when it enters the radio range of a new WLAN base station. The mobile node initiates a discovery protocol for receiving a new IP address and invites the calling host to this new address. Schulzrinne et al. consider SIP-based mobility to be less suitable for TCP-based applications. With SIP based handover, however, it is not possible to provide for a sufficiently efficient WLAN handover. It is as well not possible to provide for a robust WLAN handover. And finally, with SIP based handover it is also not possible that network providers may share network resources in between each other.
The patent application US 2003/0185172 discloses a method for supporting mobility of a mobile node between access points of a WLAN (wireless local access network). The method is initiated by moving the mobile node from a first WLAN, where the mobile node currently is connected, to a second WLAN. After moving the mobile node, a message containing the mobile nodes medium access control (MAC) address is broadcasted by a second access server of the second WLAN. A first access server of the first WLAN receives the broadcasted message and sends a message containing the Internet protocol address of the mobile node to the second access server. Data routing to and from the mobile node is then established via the second access server via the first access server to the Internet. A disadvantage of the method according to US 2003/0185172 is that a sufficiently efficient WLAN handover is difficult to achieve. It is also a disadvantage that robustness of a WLAN handover cannot be achieved. When the mobile node moves from a first network provider to a second network provider, it is a disadvantage that the mobile node has to be newly registered to the second network provider.